Observing US holidays in Korea means that you get a few Mondays off when everyone else has to work. (Of course, everything in Seoul is closed on Monday because the department stores, museums and attractions are open all weekend. It's a catch 22.)
The glitter you see on the horizon is the water. There was probably about a mile of mud before it. The tides in Incheon are huge. And the mud is full of shells so it's not something that you want to be walking through barefoot.
This is the beach that a lady sent us too after telling us that the others in the area were too muddy. But as you can tell, there was no one else on the beach with us.